This invention relates generally to the field of radar. More specifically, the present invention relates to synthetic aperture radar and moving target indication modes of such radars.
Radar has been developed to provide detection, tracking, and imaging of objects in all weather conditions using electromagnetic signals. Moving target indication is a particular mode of radar in which stationary objects are separated from objects in motion. Typically, this is accomplished by passing successive radar returns through a subtraction circuit. The returns of objects which are not moving remain the same from pulse-to-pulse, resulting in their removal through the differencing process. On the other hand, the returns of moving objects vary in phase and amplitude on successive pulses, so that the returns of targets in motion are not cancelled.
The angular resolution of a radar beam is inversely related to the physical dimensions of the radar antenna. Moving target indication is typically employed for detection and tracking functions, which require only modest radar beamwidths. At the same time, large antennas are often required to provide sufficient gain. Multiple-channel phased array antennas were developed in part to provide a wide field-of-view concurrent with fine angular resolution.
The imaging function requires a higher degree of angular resolution than detection or tracking, and therefore requires a much larger antenna. The synthetic aperture radar technique was invented to produce long virtual apertures by translating a small physical aperture in space using the motion of the host platform. As the physical aperture is moved, the signals transmitted and received by the aperture are phase-shifted and added to produce a resultant sum that is similar to that of a larger physical aperture with many elements or subarrays. The virtual aperture is M times larger than the physical aperture, where M is the number of signals integrated, and results in a corresponding improvement in spatial resolution on the ground. In strip map mode, the antenna beam of the radar system remains in a fixed angular pointing direction, and an image of the surface is formed as the beam moves over the area to be imaged due to motion of the radar platform. In spotlight synthetic aperture radar, the direction of the beam pointing changes as the platform moves to keep the beam pointing at a fixed location on the ground. Multi-channel synthetic aperture radars have been developed using a phased array antenna and multiple receiver channels.
Besides different antenna provisions, synthetic aperture radar and moving target indication have conflicting waveform requirements. Synthetic aperture radar requires a wide beamwidth waveform, whereas moving target indication employs a relatively narrowband signal. The two functions also specify differing pulse repetition frequency requirements. These different demands require that synthetic aperture radar and moving target indication be performed either sequentially or using separate systems, although in some cases moving target indication information is derived from synthetic aperture radar images.
The multi-channel radio camera was invented to provide for array distortion correction in synthetic aperture and inverse synthetic aperture radar.
The frequency diverse array (see U.S. Pat. Nos. 7,319,427 and 7,511,665 B2) was invented to provide range dependent beamforming as well as control of adaptive transmit signals for the simultaneous use of radiated energy for multiple conflicting requirements.
A limitation of the prior art is that synthetic aperture radar requires a small aperture to provide a wide beam width to allow long integration times, whereas moving target indication requires large apertures for high gain, narrow beam widths, and low antenna sidelobes which aid in clutter suppression.
Another limitation of the prior art is that moving target indication implemented using synthetic aperture radar has been constrained to along-track interferometry, and to image change detection, which requires that images be formed prior to the detection of moving targets.
Still another limitation of the prior art is that spot light and strip map modes of synthetic aperture radar cannot be operated simultaneously.
Yet another limitation of the prior art is that synthetic aperture radar and real-beam moving target indication cannot be performed simultaneously.
Still yet another limitation of the prior art is that a congested radio frequency spectrum constricts contiguous bandwidth available for high resolution imaging.